Multifrequency acoustic chamber for the agglomeration and separation of particles suspended in gaseous effluents

ABSTRACT

The objective of the present patent is an acoustic chamber for treatment of gaseous effluents containing solid and liquid micro-particles in suspension at various frequencies and high intensities. The acoustic energy agglomerates the micro-particles, thereby facilitating their separation by means of conventional systems (electrostatic filters, cyclones, etc.). The chamber has a rectangular cross section and is axially traversed by the flow of the aerosols to be agglomerated (1), while the acoustic generators, being of vibrating-stepped-plate type (2) and being able to work at different frequencies, are arranged contiguously or alternatingly on the side walls to create intensive stationary fields in the various transversal sections of the chamber. To take advantage of the emissions from both faces of the acoustic generator&#39;s radiating plate, it is proposed to locate it in between of two planar reflectors which form an angle. Likewise, for the aerosol treatment at high temperatures a refrigeration system (4) for the acoustic radiators is proposed by means of a cool air flow.

This is a continuation of international application Ser. No.PCT/ES94/00026, filed Mar. 11, 1994.

FIELD OF THE INVENTION

The present invention pertains to multifrequency acoustic chambers forthe agglomeration and separation of particles suspended in gaseouseffluents.

BACKGROUND OF THE INVENTION

Particles suspended in gases coming from industrial emissions or fromthe exhausts of internal combustion engines constitute one of thefactors with the greatest impact on atmospheric contamination. Morespecifically, the very fine particles (smaller 5 microns) which are verydifficult to eliminate with conventional separation technologiesrepresent a major health hazard because of their penetration andadhesion ability to respiratory tissue as well as for their generaltoxicity. At this time, efficient industrial emission control ofparticulate matter is only possible down to particle sizes in the orderof some microns by means of electrostatic filters.

Acoustic energy offers a new method for elimination of micron- andsubmicron-sized particles. The Spanish Patent 439.523 indicates how theapplication of a high intensity acoustic field with an appropriatefrequency in an aerosol gives raise to an agglomeration processes of theparticles, which form the aerosol, increasing their size to facilitatein this way their later precipitation or separation in a conventional orgravity collector. The process takes advantage of the effects ofvibration, hydrodynamic interaction, and entrainment which are producedbetween the particles by the action of the acoustic field.

The object of the present patent refers to an acoustic chamber fortreatment of gaseous effluents which contain solid or liquid particlesin suspension. There are some precedents of equipment of this type. TheSpanish Patent 459.523 presents a cylindrical chamber of which one endincorporates an acoustic emitter of radiating-stepped-plate type and theother one a plane reflector paralleling the face side of the emitter.The aerosol to be treated is introduced tangentially and it carries outa helicoidial course which, even though favoring the particleagglomeration, introduces load losses and provokes turbulent flow.

The European patent EP-A-488,097 presents a new chamber type ofpolygonal cross section in which the aerosol enters axially while theacoustic emitter or emitters (whose type is not specified) are locatedat an oblique angle with respect to the chamber axis. This way, theacoustic radiation is repetitively reflected on the chamber walls untilit falls, on the other end of the chamber, onto a plane reflector whichis also located at an oblique position with respect to the axis. Thisoblique incidence, however, can favor the excitation of multiple modeswhich can produce energy dispersion between the principal axial mode andthe different transversal modes. On the other hand, with the use ofseveral acoustic sources of equal frequency within the same chambersection one risks to produce negative interferences by generating theacoustic field.

Both inventions try to take advantage of the maximum axial length of thechamber and of the application of a single operating frequency. Coolingdevices for the acoustic emitters, which permit the operation with gasesat high temperatures, are proposed in none of these systems.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is a new chamber type with apolygonal cross-section such as a rectangular cross section based on theapplication of high acoustic energy levels by means of emitters such asradiating-stepped-plate type generators (European Patent EP450030A1)which treat an aerosol flow, throughout its passing-by, in directionperpendicular to the propagation (FIG. 1). In this manner, the gases tobe treated will flow in a straight line along the chamber axis,indicated by Arrow 1, while the emitters (2), which can be of differentfrequencies, will be distributed on the side walls to create intensivestationary fields which will cover the treatment chamber from end to endin all its different sections without leaving extensive zones at lowacoustic levels. The chamber walls opposing the emitters will functionas reflectors. The transducer support implements an adjustment device(3) which permits the variation of the radiator-reflector distance toachieve an optimum stationary field. By letting every acoustic emitteract on the width of the chamber (instead of on the length, as done inthe former mentioned inventions) one will need to cover smallerdistances and so one will be able to achieve a higher intensity as wellas a better homogeneity of the acoustic field, since generally one willwork in the nearfield of the emission where the beam maintainspractically parallel.

The possibility to apply different frequencies along the flow path isvery important since, because of the existence of an optimumagglomeration frequency for each particle size, the change of the sizedistribution through the process itself requires a change of thetreatment frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a first embodiment of themultifrequency acoustic chamber of the present invention.

FIG. 2 illustrates schematically a second embodiment of themultifrequency acoustic chamber of the present invention.

FIG. 3 is a sectional view of a stationary field generated by anacoustic emitter of the second embodiment of the present invention.

DETAILED DESCRIPTION

In FIG. 1, where following the present invention a scheme of theacoustic chamber for aerosol treatment is presented, it is also proposedto include a cooling system for the transducers for their use intreatment of high temperature gases. This system is based on theintroduction of a cold or ambient temperature air flow (at the level ofthe radiating plate) which acts as a gaseous curtain separating theacoustic emitter and the high temperature aerosol. This way, atemperature gradient is produced which not only serves to thermallyprotect the emitter but also, in addition, to produce a gradual effectof impedance adaptation between the radiator and the medium. Such asystem might be required for those industrial applications in which thegaseous effluents proceeding combustion processes are emitted attemperatures in the order of 150° C. or even higher.

As said before, the acoustic emitters to be used in the proposed chamberare of type "plate radiator with stepped profile." One arrangement ofthe acoustic emitters in the multifrequency acoustic chamber hasprovides emitters with a radiator plate including a front side, with theemitters disposed so as to emit directly from the front side of theradiator plate into the flow path. In the arrangement shown in FIG. 1,the emission from the plate's backside is not taken advantage of for theagglomeration process, except in the case if the mentioned plate iscompletely introduced in the interior of the treatment chamber. Anarrangement with which the mentioned backside radiation could berealized is presented in FIG. 2. The transducer (2) is located with itsaxis parallel to that of the chamber in a way that the radiating plateis situated in between of two reflector plates (5), which form an anglesuch that the forward as well as the backward radiation can be reflectedand normally directed into the gas stream which is flowing along thetreatment chamber (FIG. 3). Like this, one obtains an efficient use ofthe total radiated energy of the emitter. It is to say, however, thatthis arrangement implies major difficulties on the adjustment andrealization of an optimum stationary wave.

As an example, we can show that with 150 W power a 20 kHz transducer,located with its axis parallel to that of the chamber in between of twoplane reflectors which form a 90° angle and are positioned at a 45°angle with respect to the radiating plate, achieves, in a chamber with across section of 140×60 cm, acoustic pressure levels higher than the 150dB which are necessary for rapid generation of the acousticagglomeration phenomenon. Without taking advantage of the backsideradiation by means of reflectors, the mentioned pressure levels arereached with applied powers in the order of 200 W.

In a rectangular chamber with a cross section of 0,5×0,5 m and a lengthof about 2 m it is possible to treat aerosol flows of from 1000 to 2000m³ /h by applying four radiating plate transducers with diameters of 48cm and radiated acoustic powers in the order of 300 W/unit. Thetransducers are located as groups along one of the chamber walls oralternatingly as shown in FIG. 1. If the backside radiation is nottaking advantage of by abstaining from using angled reflectors with theradiators, the effective acoustic potential is approximately one half ofthe radiated one, that is to say in the order of 150 W/unit. This meansthat the total power applied to the aerosol on its run throughout thetreatment chamber is about 600 W. The intensity levels achieved in theinterior of the chamber are higher than 160 dB. This way, one achieves,with frequencies of 20 kHz for aerosols with initial particles from 0,2to 2 microns and concentrations from 0,1 to 4 g/m³, enlargement of anorder of magnitude. That is to say, the mean size grows from one to tenmicrons. This increase is decisive for the subsequent application of anelectrostatic filter to the aerosol flow, since these filters aregenerally efficient from 5 microns on.

What is claimed:
 1. A multifrequency acoustic chamber for agglomerationand separation of particles suspended in gaseous effluents,comprising:walls, each of said walls facing an opposed parallel wall,said walls defining a polygonal transverse cross section such thatacoustic radiation emitted orthogonally from one of said walls will fallorthogonally on a surface of said opposed parallel wall and be reflectedby said opposed wall, giving rise to the generation of a stationaryacoustic field; said chamber having an input end and an output end, saidwalls defining a flow path therebetween for the gaseous effluents fromsaid input end to said output end; a plurality of acoustic emitters foremitting acoustic radiation having a pressure level greater than 150 dBwithin said chamber, said emitters being along said flow path, each ofsaid acoustic emitters emitting radiation at a predetermined frequencyin a direction orthogonal to said walls, the emitted radiation of someof said emitters being different than others, and the emission frequencyof each of said emitters being related to the position of that emitteralong said flow path, whereby a frequency distribution of acousticradiation is provided along said flow path in relationship to the changeof size distribution of the particles in the gaseous effluents due tothe acoustic agglomeration process.
 2. The multifrequency acousticchamber as in claim 1, wherein said emitters have a radiator plateincluding a front side, said emitters being disposed so as to emitdirectly from the front side of said radiator plate into said flow path.3. The multifrequency acoustic chamber as in claim 1, wherein saidemitters have a radiator plate including a front side and a back side,and wherein said chamber further comprises plane reflectors located onopposite sides of said radiator plate of each of said emitters andforming an angle thereto such that radiation from the front and backsides of each of said radiator plates is reflected by said planereflectors and directed into said flow path in an orthogonal directionwith respect to said walls.
 4. The multifrequency acoustic chamber as inclaim 3, further comprising an adjustment means for adjusting theposition of said radiator plates with respect to said plane reflectors.5. The multifrequency acoustic chamber as in claim 1, further comprisinga cooling system, said cooling system including means for providing alaminar flow of cool air to separate the acoustic emitter from gaseouseffluents passing along the flow path.
 6. The multifrequency acousticchamber as in claim 1, wherein said emitters are placed along said wallsas contiguous units or groups.
 7. The multifrequency acoustic chamber asin claim 1, wherein said emitters are placed on alternating ones of saidwalls.
 8. The multifrequency acoustic chamber as in claim 1, whereinsaid emitters are radiating-stepped-plate acoustic emitters.
 9. Themultifrequency acoustic chamber as in claim 1, wherein said polygonaltransverse cross section is a rectangular transverse cross section. 10.The multifrequency acoustic chamber as in claim 1, wherein said emittersare distributed and positioned along said flow path to emit acousticradiation, the radiation maintaining the acoustic pressure level withinthe chamber relatively free of low acoustic level zones.
 11. Themultifrequency acoustic chamber as in claim 1, wherein the flow pathwithin the chamber is linear between said input end and said output end.12. A multifrequency acoustic chamber for agglomeration and separationof particles suspended in gaseous effluents, comprising:walls, each ofsaid walls facing an opposed parallel wall, said walls defining apolygonal transverse cross section such that acoustic radiation emittedorthogonally from one of said walls will fall orthogonally on a surfaceof said opposed parallel wall and be reflected by said opposed wall,giving rise to the generation of a stationary acoustic field; chamberhaving an input end and an output end, said walls defining a flow paththerebetween for the gaseous effluents from said input end to saidoutput end; a plurality of acoustic emitters for emitting acousticradiation having a pressure level greater than 150 dB within saidchamber, said emitters being along said flow path, each of said acousticemitters emitting radiation at a different predetermined frequency in adirection orthogonal to said walls, the emission frequency of each ofsaid emitters being related to the position of that emitter along saidflow path, whereby a frequency distribution of acoustic radiation isprovided along said flow path in relationship to the change of sizedistribution of the particles in the gaseous effluents due to theacoustic agglomeration process.
 13. The multifrequency acoustic chamberas in claim 12, wherein said emitters have a radiator plate including afront side, said emitters being disposed so as to emit directly from thefront side of said radiator plate into said flow path.
 14. Themultifrequency acoustic chamber as in claim 12, further comprising acooling system, said cooling system including means for providing alaminar flow of cool air to separate the acoustic emitter from gaseouseffluents passing along the flow path.
 15. The multifrequency acousticchamber as in claim 12, wherein said emitters are placed along saidwalls as contiguous units or groups.
 16. The multifrequency acousticunit as in claim 12, wherein said emitters are placed on alternatingones of said walls.
 17. The multifrequency acoustic chamber as in claim12 wherein said emitters are radiating-stepped-plate acoustic emitters.18. The multifrequency acoustic chamber as in claim 12, wherein saidpolygonal transverse cross section is a rectangular transverse crosssection.
 19. The multifrequency acoustic chamber as in claim 12, whereinsaid emitters are distributed and positioned along said flow path toemit acoustic radiation, the radiation maintaining the acoustic pressurelevel within the chamber relatively free of low acoustic level zones.20. The multifrequency acoustic chamber as in claim 12, wherein the flowpath within the chamber is linear between said input end and said outputend.